Noble metals such as Pt (platinum), Rh (rhodium) and Pd (palladium) supported by perovskite-type composite oxides represented by the general formula: ABO3 have been widely known as three-way catalysts which can simultaneously clean up carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) contained in emissions.
When such a noble metal is merely supported by the perovskite-type composite oxide, however, the noble metal undergoes grain growth at the surface of the perovskite-type composite oxide in use at high temperatures. To avoid this, techniques of incorporating a noble metal into a perovskite-type composite oxide as its constitutional element have been proposed.
Japanese Laid-open (Unexamined) Patent Publication No. Hei 6-100319, for example, discloses a method of producing a perovskite-type composite oxide. In this method, an
aqueous solution containing citric acid and salts of elements constituting the perovskite-type composite oxide and including the noble metals is initially prepared, and the aqueous solution is dried to obtain a complex between citric acid and the respective elements. The complex is thermally decomposed by heating at 350° C. or higher in vacuum or in an atmosphere of inert gas to obtain a precursor. The precursor is subjected to a heat treatment under an oxidative atmosphere to obtain the perovskite-type composite oxide.
Japanese Laid-open (Unexamined) Patent Publication No. Hei 8-217461 discloses a method for producing a perovskite-type composite oxide. In this method, a solution of alkoxides of elements constituting the perovskite-type composite oxide other than the noble metal is initially prepared. The alkoxide solution is hydrolyzed by adding an aqueous solution of a salt of the noble metal, and the solvent and fluid are removed to obtain a precursor. The resulting precursor is subjected to a heat treatment at 500° C. to 500° C. under an oxidative atmosphere to obtain the perovskite-type composite oxide.
Japanese Laid-open (Unexamined) Patent Publication No. 2000-15097 discloses a method for producing a perovskite-type composite oxide. In this method, a salt of the noble metal constituting the perovskite-type composite oxide is mixed with an organic polymer to obtain a colloid solution of the noble metal. The colloid solution is mixed with alkoxides of elements constituting the perovskite-type composite oxide and is then hydrolyzed to obtain a precursor. The precursor is dried and is subjected to a heat treatment to obtain the perovskite-type composite oxide.
In these perovskite-type composite oxides, the noble metals can be finely and highly dispersed therein and can maintain their high catalytic activities even in long-term use. This is because of a self-regenerative function, in which the noble metals undergo repetitive solid-solution under an oxidative atmosphere and deposition under a reducing atmosphere. These findings have been obtained in recent years.
Any of the above-mentioned methods, however, has limitations to efficiently disperse the noble metal in the perovskite-type composite oxide for a higher rate of solid-solution and to improve the catalytic activity.
The above-mentioned methods use the noble metal as an aqueous solution of a salt thereof such as nitrate, chloride, or dinitrodiammine salt and may invite an abrupt exothermic reaction in the heat treatment, which may in turn invite bubbling over of the resulting powder. To avoid this, the temperature must be gradually raised, which constitutes a significant limitation in industrial production. In addition, the heat treatment yields harmful by-products such as nitric acid, hydrochloric acid, or amines but these must be avoided for safety or hygiene.
In addition, the above-mentioned methods require considerably high temperatures in the heat treatment to form a single phase of the perovskite-type composite oxide. The heat treatment at such high temperatures, however, inevitably invites a decreased specific surface area.